1. Field of the Invention
The present invention relates to a thermal head for use in a thermographic printer, for example more particularly, it relates to a thermal head which comes into favorable touch with a thermosensitive sheet, such as inked film or heat-sensitive color developing paper, exhibits a high thermal responsiveness and establishes the optimum temperature distribution on the surface of the head and which is therefore well-suited for printing of high resolution and high quality at high speed.
2. Description of the Prior Art
In general, a thermal head comprises a substrate made of ceramics or the like, a heat accumulating member layered on the substrate, and a plurality of minute heating resistors arranged on the surfacer of the heat accumulating member, as disclosed in, e.g., IEEE TRANSACTIONS ON COMPONENTS, HYBRIDS, AND MANUFACTURING TECHNOLOGY, VOL CHMT-4, No. 1, MARCH 1981. The heating resistors are respectively provided with electrodes for feeding electric power. A protective member is layered so as to cover the heating resistors and the electrodes. The protective member consists of the two layers of an oxidation-proof layer for preventing oxidation and a wear-proof layer for preventing the wear of the oxidation-proof layer. With some materials, the protective member can serve as both the oxidation-proof layer and the wear-proof layer. In this case, the protective member is formed of a single layer.
In the printing mechanism of a thermographic printer which includes this thermal head, the heating resistor is energized via the electrodes. Upon the energization, the heating resistor generates heat in its heating portion. Via the protective member, the heat is transmitted from the printing dot portion of a head surface to a thermosensitive sheet. In a case where the thermosensitive sheet is an inked film by way of example, the heat melts the ink of an ink layer, and the ink is applied to a medium to-be-recorded such as printing paper, so as to perform printing. Besides, in a case where the thermosensitive sheet is a heat-sensitive color developing paper by way of example, the heat is transmitted to a color developing layer, which develops a color so as to perform printing. Upon completion of the printing, the heating resistor is deenergized and is sufficiently cooled to the degree at which no printing is possible. Thereafter, the relative position between the thermal head and the medium to-be-recorded is shifted to the next printing position (usually, a position shifted by one dot), whereupon the series of printing operations described above are repeated.
In order to realize high speed printing, accordingly, it is required that the thermal responsiveness of the head is high, namely, that the heat generated by the heating portion of the heating resistor is quickly transmitted to the printing dot portion to raise the temperature of the dot portion up to a point necessary for melting the ink layer or for causing the heat-sensitive color developing paper to develop a color and that the heating resistor is thereafter cooled quickly. From the viewpoint of the printing quality, it is desirable that only the temperature of the printing dot portion on the heating portion rises uniformly and that the temperature of the surrounding head surface including the adjacent dot portions remains unchanged.
In general, the printing density depends greatly upon the contact pressure between the printing dot portion and the thermosensitive sheet.
More specifically, in a case where the contact pressure between the printing dot portion and the thermosensitive sheet is not higher than a predetermined value, the printing density increases with the contact pressure, and in a case where the contact pressure exceeds the predetermined value, the printing density becomes constant irrespective of the contact pressure. The shape of the surface of the thermal head accordingly needs to be such that the contact pressures between the printing dot portions and the thermosensitive sheet are uniformly distributed within, at least, the printing dot portions.
Usually, the prior-art thermal head has the printing dot portion lowered stepwise with respect to the head surface. For this reason, the contact pressures between the printing dot portion and the thermosensitive sheet do not become uniform. Particularly within the printing dot portion which is very important for the printing quality, the outer side has a lower contact pressure. At the end part of the printing dot portion, therefore, a gap arises between the printing dot portion and the thermosensitive sheet. As a result, the area of a printed dot becomes smaller than that of the printing dot portion, and the printed dot is not clearly demarcated from the surrounding dots. Moreover, the pressing force between the thermal head and the thermosensitive sheet fluctuates inevitably on account of the structure wherein the printing is repeated while the thermal head and the thermosensitive sheet are moving relatively. The fluctuation of the pressing force has incurred a fluctuation in the size of the gap between the printing dot portion of the head and the thermosensitive sheet, that is, a fluctuation in the size of the printed dot, resulting in the degradation of the picture quality. In addition, the inferior contact state between the printing dot portion of the thermal head and the thermosensitive sheet as described above increases the contact thermal resistance between the two. This has caused a great temperature difference between the printing dot portion and the thermosensitive sheet. Accordingly, the temperature of the printing dot portion has needed to be very high in order to melt the ink of the ink layer in the case of the inked film as the thermosensitive sheet or to develop a color in the case of the heat-sensitive color developing paper. Besides, the heat generated by the heating portion of the heating resistor and conducted within the protective layer toward the printing dot portion propagates to the surroundings due to the great contact thermal resistance between the printing dot portion and the thermosensitive sheet, so that it raises the temperature of the head surface around the dot portion including the adjacent printing dot portions. This has incurred such degradation of the printing quality that the printed dots are not clearly demarcated or that they spread widely.
Although the various disadvantages mentioned above are somewhat improved by increasing the pressing force between the thermal head and the thermosensitive sheet, the protective member wears off heavily to shorten the lifetime of the head. On the other hand, when the pressing force is too great, there occurs a phenomenon called pressure transfer or pressure color development in which the ink is transferred to the paper or the heat-sensitive color developing paper develops a color without the application of heat.
As a measure intended to improve such disadvantages, though it relates to a thermal pen, an example in which printing dot portions are made of diamond and in which the diamond is protruded above a head surface has been disclosed in the official gazette of Japanese Utility Model Registration Application Publication No. 58-13703. In this structure, however, the gradient of a contact pressure within the printing dot portion is rather greater than in the prior-art structure, and the contact area between the printing dot portion and the thermosensitive sheet differs greatly depending upon the pressing force between the two, so that the printing quality has been similarly low.
In the thermal head of the prior-art structure, the protective member is made of a material whose thermal conductivity K is as inferior as approximately 10.sup.-2 -10.sup.-3 cm.sup.2 /s (for example, SiO.sub.2 or Ta.sub.2 O.sub.5), and since it endures wear and serves for preventing the oxidation of the heating resistors as well as the electrodes, it is formed at a uniform thickness which is approximately 5-10 .mu.m. Therefore, the thermal resistance between the heating portion of the heating resistor and the printing dot portion of the head surface becomes very high, which has caused a great temperature difference between the heating portion and the printing dot portion. Accordingly, the temperature of the heating portion needs to be very high in order that the temperature of the printing dot portion of the head surface may be raised up to a point required for printing. In order to perform high speed printing with such thermal head, the temperature of the printing dot portion of the head surface needs to be raised up to the predetermined point in a short time. Therefore, input power to the heating resistor increases, and the temperature of the heating resistor becomes higher than in case of low speed printing, so that the head might be destroyed. Also for cooling after the cutoff of the input power, a long time is naturally required. Thus, enhancement in the speed of the printing has been limited.
A further disadvantage has been that, since the thermal resistance from the heating portion of the heating resistor to the printing dot portion of the head surface is high, much heat leaks to the surroundings, so the greater part of the input power to the heating resistor is not utilized for printing.